This invention relates to the use of certain Group 3 metal complexes to form ethylene homopolymers in high yield. More particularly, this invention relates to such processes wherein the metal complexes have a bridged ligand structure, such that the metal is linked to a cyclopentadienyl group (which optionally may be substituted as explained hereinafter) via a .eta.-bond and also linked via a divalent ligand group attached to both the cyclopentadienyl group and the metal.
Certain Group 3 metal complexes (other than those based on scandium), and methods for their preparation are disclosed in U.S. Application Ser. No. 545,403, filed Jul. 3, 1990 (EP-A-416,815) pending. The complexes, which possessed a bridged structure, were disclosed as being active polymerization catalysts when combined with activating cocatalysts such as aluminoxanes. In U.S. Pat. No. 5,064,802, cationic metal complexes including complexes of Group 3 metals including such complexes having a bridged structure were disclosed. Such cationic complexes result by combination of metal complex precursors with a cation forming activating cocatalyst, particularly an ammonium salt of tetrakis(pentafluorophenyl)borate. In Shapiro et al., Organometal., 1990, 9, 867-869, scandium complexes were disclosed which were coordinated Lewis base adducts, having relatively poor catalytic properties. The reference reported that small quantities of oligomeric or polymeric propene, 1-butene and 1-pentene addition products were formed with lengthy reaction times using as a catalyst the dimeric hydride derivative, bis( (t-butylamido)(tetramethyl-.eta..sup.5 -cyclopentadienyl)dimethylsilanescandium ditrimethylphosphine) dihydride.